Neurostimulation systems are devices that generate electrical pulses and deliver the pulses to nerve tissue to treat a variety of disorders. Spinal cord stimulation (SCS) is an example of neurostimulation in which electrical pulses are delivered to nerve tissue in the spine for the purpose of chronic pain control. While a precise understanding of the interaction between the applied electrical energy and the nervous tissue is not fully appreciated, it is known that application of an electrical field to spinal nervous tissue can effectively mask certain types of pain transmitted from regions of the body associated with the stimulated nerve tissue. Specifically, applying electrical energy to the spinal cord associated with regions of the body afflicted with chronic pain can induce “paresthesia” (a subjective sensation of numbness or tingling) in the afflicted bodily regions. Thereby, paresthesia can effectively mask the transmission of non-acute pain sensations to the brain.
Neurostimulation systems generally include a pulse generator and one or several leads. The pulse generator is typically implemented using a metallic housing that encloses circuitry for generating the electrical pulses. The pulse generator is usually implanted within a subcutaneous pocket created under the skin by a physician. The leads are used to conduct the electrical pulses from the implant site of the pulse generator to the targeted nerve tissue. The leads typically include a lead body of an insulative polymer material with embedded wire conductors extending through the lead body. Electrodes on a distal end of the lead body are coupled to the conductors to deliver the electrical pulses to the nerve tissue.
There are concerns related to the compatibility of neurostimulation systems with magnetic resonance imaging (MRI). MRI generates cross-sectional images of the human body by using nuclear magnetic resonance (NMR). The MRI process begins with positioning the patient in a strong, uniform magnetic field. The uniform magnetic field polarizes the nuclear magnetic moments of atomic nuclei by forcing their spins into one of two possible orientations. Then an appropriately polarized pulsed RF field, applied at a resonant frequency, forces spin transitions between the two orientations. Energy is imparted into the nuclei during the spin transitions. The imparted energy is radiated from the nuclei as the nuclei “relax” to their previous magnetic state. The radiated energy is received by a receiving coil and processed to determine the characteristics of the tissue from which the radiated energy originated to generate the intra-body images.
Neurostimulation systems are designated as being contraindicated for MRI, because the time-varying magnetic RF field causes the induction of current which, in turn, can cause significant heating of patient tissue due to the presence of metal in various system components. The induced current can be “eddy current” and/or current caused by the “antenna effect.”
“Eddy current” refers to current caused by the change in magnetic flux due to the time-varying RF magnetic field across an area bounding conductive material (i.e., patient tissue). As shown in a simplified form in FIG. 1, the time-varying magnetic RF field induces current within the tissue of a patient that flows in closed-paths. When conventional pulse generator 103 and conventional implantable lead 104 are placed within tissue in which eddy currents are present, implantable lead 104 and pulse generator 103 provide a low impedance path for the flow of current. As depicted in FIG. 1, electrode 102 provides a conductive surface that is adjacent to current path 101. Electrode 102 is coupled to pulse generator 103 through a wire conductor (not shown) within implantable lead 104. The metallic housing (the “can”) of pulse generator 103 also provides a conductive surface in the tissue in which eddy currents are present. Thus, current can flow from the tissue through electrode 102 and out the metallic housing of pulse generator 103. Because of the low impedance path and the relatively small surface area of electrode 102, the current density in the patient tissue adjacent to electrode 102 can be relatively high. Accordingly, resistive heating of the tissue adjacent to electrode 102 can be high and can cause significant tissue damage.
Also, the “antenna effect” can cause current to be induced which can result in undesired heating of tissue. Specifically, depending upon the length of the stimulation lead and its orientation relative to the time-varying magnetic RF field, the wire conductors of the stimulation lead can each function as an antenna and a resonant standing wave can be developed in each wire. A relatively large potential difference can result from the standing wave thereby causing relatively high current density and, hence, heating of tissue adjacent to the electrodes of the stimulation lead.